The present invention relates in general to liquid crystal displays (LCDs), and in particular to normally white, supertwist nematic liquid crystal displays of reflective type.
Reflective liquid crystal displays are widely used in mobile systems, since they feature very low energy consumption, small size, low weight, and are convenient for use outdoors. Reflective displays with a single polarizer have good potential for reaching high brightness, enhanced contrast ratio and low energy consumption.
Supertwist nematic liquid crystal displays (STN displays) of reflective type with single polarizer possess potentially high brightness and good color rendition capacity. STN displays are characterized by large twist angles of nematic liquid crystal (LC) directors (approximately from 180° to 260°) as compared to regular twist nematic liquid crystal displays (TN displays). STN displays provide voltage-contrast characteristics with sharp cutoff, which are required to obtain high multiplexing ability and contrast ratios. STN displays feature extraordinarily high resolution ability and small pixel size, which enhance information capacity of such displays in displaying numerical (symbolic) information as well as in displaying images, for example photographs. Consequently, STN displays feature excellent image quality, as compared to regular passive-matrix TN-displays. These advantages are especially pronounced in large displays with high multiplexing level. STN displays cost low, require low operating voltage and feature low energy consumption.
A normally white, supertwist nematic liquid crystal display of reflective type is described in “TFT/LCD Liquid-Crystal Displays Addressed by Thin-Film Transistors” by Toshihisa Tsukada, Vol. 29, Gordon and Breach Science Publishers, p. 153. The known STN-display contains a layer of nematic liquid crystal. The dependences of the voltage-contrast characteristic on the LC director twist angle in the range from 210 to 330° are presented. It has been shown that the dependence of the contrast ratio on the applied control voltage is sensitive at the twist angle of 240°. However, the optimum direction of the transmission axis of the polarizer and the optimum values of the product dΔn of the nematic layer thickness (d) and the difference of the refraction indexes (optical anisotropy) Δn, which provide maximum values of the contrast ratio and brightness, have not been determined.
A known normally white, supertwist nematic liquid crystal display of reflective type is described by Shin-Tson Wu, Deng-Ke Yang, “Reflective Liquid Crystal Displays”, John Wiley & Sons, Ltd., p. 9. This known STN display consists of a glass plate, which is coated with a layer of aluminum to create a reflecting surface (mirror), and a layer of nematic liquid crystal, above which there is a second glass plate. Polarizing and scattering layers are formed on the second glass plate. One of the drawbacks of this known STN display is that the optimum direction of the optical transmission axis of the polarizer, which provides maximum contrast of the display, is not known.
A known twist nematic liquid crystal reflective display comprising a liquid crystal and two polarizers with an LC director twist angle of 45° is described by S. -T. Wu, D. -K. Yang, Reflective Liquid Crystal Displays, 2001 by John Willey & Sons Ltd, p. 108. This design requires two polarizers, one of which is placed between the liquid crystal and the mirror. The drawback of this design is image parallax, which prohibits the display from being used in applications that require high resolution. Since a second polarizer is necessary, the design cannot be simplified by combining the functions of a mirror and an electrode in a single element. Such display has relatively low brightness and small viewing angle and contrast, etc.
A possible design of twist nematic liquid crystal reflective display comprises a liquid crystal between two electrodes, a phase compensator and one polarizer. One of the two electrodes in this design is transparent, while the second electrode has good reflective ability and functions as a mirror at the same time. The phase compensator (or compensator) represents a phase shifting plate, which provides a phase delay of π/2. The compensator eliminates the need for a second polarizer. Due to dual birefringence in the absence of the second polarizer, the light becomes elliptically polarized after it travels twice through the liquid crystal. Therefore, the compensator changes the elliptical polarization to linear, which provides image contrast. The drawback of this design is that it is impossible to precisely transform elliptical polarization into linear polarization for all wavelengths at any selected operating voltage across the liquid crystal.
This drawback manifests itself in relatively low image contrast. Another drawback of this design is the inclusion of the compensator, which complicates the design of the display.
Another twist nematic liquid crystal reflective display comprises a liquid crystal, two electrodes, a phase compensator and a single polarizer. The design of this display suggests using a phase compensator to obtain high quality image without parallax. The phase compensator can essentially be any material suitable for use in displays and featuring birefringence. The phase compensator installed between the polarizer and liquid crystal allows making the second electrode reflective, thereby simplifying the design. The design requires selection of special parameters of the liquid crystal and compensator according to conditions that are difficult to satisfy in the entire visible region of the spectrum. This manifests itself in the following drawbacks of the design. First, either distortions of color rendition are possible, or using different voltage for the blue, red or green colors may result in a more complex design of display. Second, since the phase compensator and the liquid crystal have to have weak dispersion, optical parameters of which are interrelated, material selection for the design is hindered.
There is a twist nematic liquid crystal reflective display comprising a front polarizer, a phase compensator and a liquid crystal. In order to increase the contrast and brightness of the image, the following parameters are selected: the angle between the transmission axis of the polarizer and the alignment direction of the LC director on the front surface of the liquid crystal, the optical anisotropy of the liquid crystal and the phase compensator, as well as the twist angle of the liquid crystal.
The drawback of above mentioned displays using a phase compensator plate is that the image is very sensitive to the thickness of the liquid crystal and the phase compensator, i.e., sensitive to the precision of fabrication. Besides, correction of image distortions related to fabrication tolerances complicates the design and imposes special requirements to the properties of the liquid crystal.
Another twist nematic liquid crystal reflective display comprises a single polarizer and does not contain the phase compensator. To obtain the best brightness, contrast and color rendition, this design suggests using optimized values of the LC director twist angle, the angle between the optical axis of the polarizer and the alignment direction of the director at the surface of LC closest to the polarizer, and the optical path difference between the ordinary and extraordinary rays in the liquid crystal. The drawback of this design is the uncertainty of the mentioned parameters, which complicates the optimization.
Another twist nematic liquid crystal reflective display can comprise a polarizer and liquid crystal. To obtain the best brightness, contrast and color rendition, and low sensitivity to the variations of thickness of the liquid crystal cell, this design suggests optimization of the LC director twist angle, the angle between the optical transmission axis of the polarizer and the alignment direction of the director of LC on the surface closest to the polarizer, and the optical anisotropy of the liquid crystal. One of the drawbacks of the display is, first, the high sensitivity to a voltage level of a switched off condition, i.e., the voltage which defines transmission state of the display. It also does not maintain the achromaticity when it transits from the “black” state to the “white” state. Moreover, the level of contrast and brightness is insufficient.
Another display uses the angle between the transmission axis of the polarizer and the orientation of LC directors as another parameter of optimization. This parameter is varied to obtain high contrast, brightness and achromaticity, in addition to the twist angle of the liquid crystal and the optical path difference. Optimization is performed with the specially designed mathematical method (H. S. Kwok, Parameter space representation of liquid crystal display operating modes, J. Appl. Phys. 80 (7), p. 3687, 1996). The drawback of this display is, first, the wide range of optical path differences proposed by the authors. This display suffers from poor stability within the region of the suggested angles. In other words, the proposed solution does not take into account the high sensitivity of the mixed regime of operation of the liquid crystal to the precision of fabrication of displays. Another drawback is the non-standard values of the twist angles. The small values of twist angles do not provide multiplexing ability of the display.
Another known normally white, supertwist nematic liquid crystal display of reflective type comprises a reflector, a single polarizer and a special retarder, twist-retarder (TR). The drawback of this display is that the display contains a retarder layer, and that the optimum orientations of the optical transmission axis of the polarizer which provide the maximum contrast of the display are not known. The presence of the retarder in the display results in additional losses of light transmission, complicates the design of the display, increases its size, and raises the manufacturing cost of the display.
There is a method of fabrication of the twist nematic liquid crystal reflective display with optimized LC director twist angle and its optical anisotropy. Despite the author's claims of generalized displays with a single polarizer, these displays are not truly single polarizer displays. The beam splitter used in the display in fact plays the role of two polarizers. This is because the polarizing beam splitter transmits one polarization state, but reflects the orthogonal one. Thus, if a sheet polarizer is used, it is necessary to use additional retarders to obtain a black appearance in ON state (or OFF state for normally-black operation mode). The use of the retarder complicates the design and makes additional problems when internal polarizers made of a thin crystalline film (TCF) are used. Another disadvantage is the fact that the author assumes the “black” state (or “bright” state for normally-black mode) corresponding to the homeotropic distribution of the LC director. It is not the case for STN designs, where to obtain high multiplexing ability the black state does not correspond to the truly homeotropic distribution of the LC director, because it may lead to high voltage difference between OFF and ON states, which is incompatible with a high multiplexing level. To make a real optimization, it is necessary to solve not only the optical problem, but also the problem of in-field behavior of the LC director.